1. Field of the Invention
This invention relates to the art of submersible electric motor driven centrifugal pump units having inducers in the pump inlet to develop a desired suction head for the main pump impeller under low submergence conditions and is particularly concerned with the driving of the inducers independently of the main impeller to develop the desired suction head to maintain full pump flow even during the pumping of fluid from the bottom of a tank and without drawing excess power or developing undesirable thrusts.
2. Prior Art
Inducer equipped submersible electric motor driven pump units adapted for the pumping of cryogenic fluids or fluids at their boiling points are known, for example, in my prior U.S. Pat. Nos. 3,304,877 issued Feb. 21, 1967; 3,369,715, issued Feb. 20, 1968; 3,652,186 issued Mar. 28, 1972; and 3,764,236, issued Oct. 9, 1973. In the units of these patents four blade inducers were mounted on the main pump shaft in the pump inlet ahead of the main pump impeller and, of course, had to be driven at the same speed as the impeller requiring excessive driving power, developing excessive thrust loads, and producing large amounts of vapor which was not reabsorbed or condensed before it entered the main impeller and thereby impaired the impeller capacity and reduced the suction head for the pump. These units were unable to develop a required suction head under low submergence conditions and excessive time was needed to pump out the last few feet or meters of cargo. In tanker ships such extra time at the discharge docks is costly and in some instances, such ships had to be moved before full cargo discharge. Since the impeller was forced to rotate at pump speed even under sufficient submergence conditions for maintaining a good suction head, it produced a head greatly in excess of that needed for the main impeller and in so doing, excess power was consumed and undesirable thrust was developed.